This invention relates to a rigid holder for mounting a plurality of cutting elements in a drag type rotary drill bit, and more particularly to a rigid integral holder or carrier for a plurality of polycrystalline diamond compact (PDC) cutting elements mounted on a unitary holder which is secured directly to the exterior face of the drill bit.
Heretofore, drag type rotary drill bits have been provided with a plurality of separate polycrystalline diamond compact (PDC) cutting elements, but normally each PDC cutting element has been mounted on a separate stud secured to a separate blade or secured within pockets formed directly on the exterior of the drill bit, such as, for example, by brazing, welding, or press fitting within a recess in the face of the bit. In some instances, separate PDC cutting elements without studs have been mounted directly on the drill bit face.
The two main bit problems encountered in drilling which tend to result in decreased penetration of a formation are deterioration or wear of the cutting elements resulting primarily from heat degradation, and "balling" of the bit face. "Balling" or sticking is caused by a sticky formation, such as sticky shales or similar formations having a large percentage of clays, adhering to the cutting face of the bit. This may occur in certain formations where the hydraulic action of drilling mud is inadequate, or where hydraulic passages in the bit may be poorly designed and result in an inefficient flow of mud across the face of the bit. It is noted that for drilling offshore the continental United States, water base muds are normally employed as government regulations generally render the use of oil base muds cost prohibitive. The use of water base muds may result in substantial "balling", particularly when drilling in sticky shales or similar formations.
It is noted that a worn PDC type bit normally forms wear flat surfaces adjacent the diamond cutting edges of the cutting elements and this causes a reduction in the penetration rate of the bit as the cutting edges do not penetrate as deeply into the formation. As the cutting elements wear, increasingly larger wear flats are formed adjacent the cutting edges. The increased size wear flats require an increasingly higher weight on the bit to maintain a constant rate of penetration into the formation thereby resulting in a higher torque requirement for rotating the bit and in a higher heat generated in the cutting elements.
The stud or support on which the PDC cutting element is mounted sometimes fractures or shears upon continued use, such as when the bit is suddenly pulled off-bottom, or if a bit bounce occurs. The cutting elements are susceptible to thermal shock as a result of rapid cooling by water or drilling mud in addition to unfavorable temperature gradients that develop combined with the sudden removal of the compressive stresses due to bit weight. This causes tension on the supports for the cutting elements. It is noted that the compressive and tensile stresses throughout the cutting elements and their supports are significantly increased under conditions in which the wear flat temperatures are around three hundred and fifty (350) degrees Centigrade. This, of course, increases the possibility of shearing or breakage of the support studs on which the PDC cutting elements are mounted. Also, the greater the projection of the support for the PDC cutting elements from the adjacent face of the drill bit, the greater the amount of stress provided on the supports from the cutting action which likewise will increase the possibility of breakage or shearing of the supports for the PDC cutting elements.
It has been found that the cutting element develops a significant wear flat by the time fifty percent (50%) of its useful life has been expended. Thus, PDC cutting elements should be designed to provide a minimal wear flat for effective operation. To maximize the life of a PDC cutting element, the rotary drill bit should be operated under conditions so that thermally accelerated wear does not occur and to reduce such wear, the rotary drill bit should be operated at a rotary speed and bit weight which does not cause the thermal wear effects to become critical at over around three hundred and fifty (350) degrees Centigrade.
Heretofore, blades have been provided on some drag type rotary drill bits having a plurality of PDC cutting elements thereon. For example, U.S. Pat. No. 4,499,958 dated Feb. 19, 1985, shows a drag type rotary drill bit with a plurality of cutting blades welded to the face of the drill bit with each cutting blade having a plurality of studs mounted thereon, each stud having a PDC cutting element on its projecting end. However, the studs are spaced a substantial distance from each other along the length of the cutting blade and each stud or support has only a single PDC cutting element thereon.
Also, U.S. Pat. No. 4,440,246 dated Apr. 3, 1984 shows a rotary drill bit with a wedge-shaped cutting member defining two PDC cutting faces formed from cutting elements extending generally at right angles to each other and mounted on a stud or carrier member secured to the bit body for providing a ploughing action against the formation.
Various other references, such as exemplified by U.S. Pat. No. 4,429,755 dated Feb. 7, 1984, show drag type rotary drill bits with stud mounted PDC cutting elements projecting from the outer face of the drill bits and arranged in various patterns on the face of the drill bit. The studs are normally secured on the face of the drill bit by brazing, welding, or press fitted within openings or recesses along the face. Various types of nozzles for drag type drill bits are illustrated in U.S. Pat. Nos. 4,303,136 dated Dec. 1, 1981 and 4,452,324 dated June 5, 1984.